The basic factors in retaining nutrients in forms available for use by plants common in natural soils include: (1) organic compounds subject to enzymatic breakdown which release plant nutrients, generally in ionic form; (2) ion exchange sites on colloids of alumino-silicates and, to some extent, on phenolic and carboxylic sites of organic colloids; (3) inorganic salts with some degree of solubility based on particle size, pH, concentrations, complementary ions, and other facets of the microenvironment; and (4) the soil solution, which bathes the soil solids and plant roots allowing cations and anions to freely exchange from one to the other.
The polyurethane plant growth medium of the present invention employs all four of the preceding principles and factors common in natural soils together with unique mechanisms that allow anion retention through the inductive effect of tertiary amine groups in the foam matrix (Factor 5), and the intimate relation of the essential plant nutrients with the polyurethane cells films which, in part, envelop the nutrient filler particles thus creating a semi-permeable membrane which responds to osmotic pressures due to ionic-molecular concentration gradients in the external solution thus providing an additional means for meeting plant nutrient requirements and fortifying the influence of the five previously-enumerated factors (Factor 6).
Other efforts have been made in the past to provide a suitable matrix or "container" for mechanized or automated propagation and growth of seedlings and cuttings, particularly in large scale agricultural, horticultural and reforestation operations. For example, there has been a need to provide automated planting and growing of conifer and hardwood seedlings and outplanting of such seedlings in reforestation, to extend the planting season to feasible limits, as against the present one or two months of outplanting possible with bareroot planting, thus making more efficient use of labor and facilities. This is a major problem in that it is necessary to replant at least one billion trees a year at present to provide adequate reforestation in the United States of America alone. Similarly, containerization is held to be beneficial for other plants, for example, flowers, vegetables, grasses and the like, both in the United States and in other countries.
One approach, in an attempt to solve this problem, has been to provide a wide range of containers, for example: paper tubes, plastic tubes, plastic webbing, and plastic and paper tray molds filled with various potting mixtures for seed germination and plant growth. These have not always been satisfactory because, among other reasons, these containers confine the root system during the early stages, which often has an effect on future growth characteristics. Particularly, in the case of trees, the lack of lateral root growth during the confinement period results in a circular type of root growth of the seedling when planted in the ground, and hence minimum lateral support for the tree and, in time, girdling may cause premature death. Also, these types of container systems are relatively expensive and have a relatively high gross weight, both wet and dry. Moreover, some plastic tubes fail to split open as a result of root pressure and, in some soils and climatic conditions, while some paper tubes disintegrate rapidly, others remain intact months after planting. The plastic tray molds filled with soil are not suitable for automatic planting in that such an unconsolidated medium must depend entirely upon root proliferation for cohesion and this is rarely achieved.
Another approach to the problem is the self-contained medium such as compressed peat moss, various compressed and molded cubes and the like (BR-8, Kys Kube), but these do not always hold together well, may lack flexibility of design, are subject to waterlogging, and generally are insufficiently supplied with nutrients.
In the home consumer trade there is a need for a plant growth medium that can be used for a variety of growth purposes which releases nutrients slowly over growing periods. There is also need for such a plant growth medium which can be used in a variety of physical forms or ground and used by itself or in potting mixtures particularly in view of the problems in obtaining adequate supplies of peat and peat of desired and consistent specification.
The present invention is directed to an improved nutrient plant growth medium which advantageously has the cation exchange capacity of good natural soils, as well as the additional factors previously mentioned, by the use of relatively inexpensive components and without the necessity of separately making an expensive nitrated nutrient ion exchange resin and then incorporating it into the foamed matrix.